1. Field of the Invention
This invention relates to a press apparatus which connects a bed and a ram by lever links and pull links, and causes the reciprocating rotary motion of the lever links and pull links by a driving device to move the ram up and down, thus eliminating the necessity for a frame having a rigid structure and providing high dynamic accuracy and high resistance against an eccentric load.
2. Description of the Prior Art
Recently, higher machining accuracy and higher productivity for press work have been increasing.
In other words, since multiple-stage dies have found a wider application due to automation by robot and progressive forming, a press having higher resistance against an eccentric load and having higher dynamic accuracy has been required to cope with such changes. However, the presses that presently satisfy these requirements are large in scale and expensive.
Typical frames of the presses have a C-shape such as shown in FIG. 18 or a gate-shape such as shown in FIG. 19, each of which has a rigid structure. The C-shaped frame provides lower press force and lower machining accuracy than the gate-shaped frame. Therefore, the gate-shaped frame is used for machining requiring high press force and high machining accuracy.
In either of these frames, when the press force and its reaction act at the time of machining, a bed portion 102 and a crown portion 104 undergo deformation as represented by the two-dot-chain line in the drawings.
Upper and lower dies of the press are fitted to the ram portion 106 and the bed portion 102, respectively. The press force and its reaction act as a bending moment on the bed portion 102 and the ram portion 106 at the time of machining, and the bed portion 102 and the ram portion 106 undergo deformation, which in turn, cause deformation of the dies fitted thereto. If the dies undergo deformation, machining accuracy drops and the service life of the dies is shortened.
The bending deformation resulting from the bending moment among the deformations of the press frame exerts particularly adverse influences on machining accuracy and service life of the metallic mold. Therefore, its bending deformation must be minimized.
Rigidity or stiffness of the frame having the rigid structure must be improved in order to reduce its bending deformation. To improve stiffness of the frame, the section modulus of the bed portion 102, crown portion 104, ram portion 106, etc, that constitute the frame must be improved. To attain this object, it is most effective to increase the height of each of these members.
If the height of each of the bed portion 102, crown portion 104 and ram portion 106 constituting the frame is increased, however, the total length of the frame is increased, and a large-scale press cannot be installed in a workshop unless part of its bed portion is embedded by boring a pit. In addition, the increased length of the press cause, the total weight of the press increase. Besides these disadvantages, it becomes difficult to assemble the large scale structures, such as the bed portion 102, the crown portion 104, the ram portion 106 and the upright portion 108 with a high level of accuracy while properly maintaining the respective horizontal and vertical orientations. Eventually, this results in an increase in the cost of the press apparatus.
As shown in FIG. 20, if a multiple-point hydraulic press which is highly resistant to the eccentric load is provided by fitting a plurality of hydraulic cylinders 110 to the crown portion 104 in order to move the ram portion 106 up and down, a synchronization mechanism for synchronizing the operations of the hydraulic cylinders 110 becomes indispensable. Such synchronization mechanism is complicated in construction and is expensive.
FIG. 21 shows a crank press of a mechanical system, which includes a crank rod 112 and a plunger 114 connected together by a connecting rod 116. A ram portion 106 is fitted to the plunger 114 and the plunger 114 is caused to slide inside a plunger guide 118. This plunger guide 118 is arranged to remove a horizontal component (Fx) of the press force (F) and to improve dynamic accuracy. However, this crank press involves the problem that the press is large in scale and expensive, due to the provision of the mechanism for removing the horizontal component of the press force by the plunger and the plunger guide is disposed.